During fabrication of integrated circuit (“IC”) masks, for example, critical dimension (“CD”) controls are implemented in stages to ensure dimensions such as spacing and line width are maintained within predefined specifications. Currently, to identify CD errors, CD measurements are generally obtained using equipment such as a critical dimension scanning electron microscope (“CD-SEM”). CD-SEM uses a scanning electron microscope to take top-down images of pattern features and extract dimension information from those images.
The fabrication environment in which CD-SEM equipment is used is replete with sources of electro-magnetic interference (“EMI”), the effects of which negatively impact the CD-SEM, resulting in blurred images and fluctuation of the measurement spot. Clearly, this has a negative impact on quality control (“QC”). Currently available CD-SEMs provide some level of EMI shielding. For example, at least one commercially-available CD-SEM has a cover made of iron (“Fe”) having a thickness of 1 mm; this provides EMI shielding of approximately 3 milligauss (mG). Such a cover, however, it is generally ineffective against the wide range of EMI (e.g., high- and low-frequency) originating from various sources within a fab at levels of 10-15 mG.
In view of the foregoing, what is needed is an improved method and system for shielding CD-SEM equipment from EMI within a fab.